So is there a down side.
 

At least for a SC with SSI, is there a downside to 20/21's or 964 Cams as compared to the stock SC cam? Why didn't Porsche use these if the performance is much better?

Just trying to understand the total picture. Good discussion regardless.

The ssi's are from the pre smog era. So the answer there is obvious. The cams (964) were developed to work with motronic controlled engines. I don't know of anyone that did a full environmental shed test of a SC equipped with these cams to see if they would pass smog specs too. I seriously doubt that they would; cis just isn't that "smart" to take care of cold start/ hot running in passing smog with a more radical camshaft design.
so in a word as to why Porsche did not do it? Smog

Thanks TRE Cup. So I guess if I live in a state where there are no smog tests, I should consider one of these cams. I thought there may be some issues with rough idle, worse gas mileage, or something else like that.

Thank God for the big three here in MI NO Smog Here!!